Support the equivalence of Attributes generic parameter in ActionReturn for Attachment

[vnphanquang]

Before Svelte attachment, Svelte action allows specifying additional attributes (including custom event listeners), on the element it is placed on:

svelte/packages/svelte/src/action/public.d.ts

Lines 27 to 38 in 5912754

	export interface ActionReturn<
	Parameter = undefined,
	Attributes extends Record<string, any> = Record<never, any>
	> {
	update?: (parameter: Parameter) => void;
	destroy?: () => void;
	/**
	* ### DO NOT USE THIS
	* This exists solely for type-checking and has no effect at runtime.
	* Set this through the `Attributes` generic instead.
	*/
	$$_attributes?: Attributes;

For example:

<script lang="ts">
  import { type ActionReturn } from 'svelte/action';

	interface ClickOutsideAttributes {
		musthave: string;
		onclickoutside?: (event: CustomEvent) => void;
	}

  function clickoutside(node: Element): ActionReturn<void, ClickOutsideAttributes> {
		/* ... */
		return {};
	}
</script>

<dialog use:clickoutside musthave="something">
	<!-- ... -->
</dialog>

As one would expect, the editor tooling will pick up onclickoutside as optional, and musthave as required - a nice ergonomics for element enhancement.

I am missing this handy feature when using attachment, especially as a lib author. Perhaps we can support a similar pattern?

Related artifacts:

• (feat) enable actions to enhance typings on applied element language-tools#1553
• feat: attachments #15000
• [TypeScript] Action Type #6538

🤞

[brunnerh]

Maybe this should just be a feature request issue?

[vnphanquang]

Yeah thanks. I was thinking perhaps it's helpful to gather people's thoughts here first before spamming the issue thread.

[brunnerh]

Discussions tend to suffer from lower visibility, so chances are that you don't get much feedback at all.
If you already have something concrete in mind, I think it is fine to directly open an issue.

[vnphanquang]

That makes sense, will do when I find some time.

[vnphanquang]

Let's move over to #16678